The present invention relates to antennas and, more particularly, to receive phased array antennas.
A phased array receiving antenna is comprised of an array of individual antenna and electronic phase shifter elements typically arranged in a planar array that is adapted to receive an electromagnetic signal. Adjusting the phase shift and/or delay of a received signal through each of the antenna and delay elements and summing the signals enables the antenna to be electronically steered. Accurate electronic steering of the antenna requires that the relative phase shift and/or delay through each of the antenna and delay elements be accurately known and adjusted. In narrow band phased array receiving antennas it is important that the signals be in-phase when they are summed. In wide band phased array antennas, both the phase and group delay of the received signals must be the same.
In severe temperature environments, encountered in arctic and space environments, for example, it is difficult to maintain the phase accuracy of the elements without calibration. Existing calibration systems use a calibrated beacon to transmit a calibration signal to the array, or transmit a calibration signal in one direction down a distribution cable to the inputs of each antenna and delay element of the antenna array. The relative phase and/or delay of this calibration signal through the antenna and delay elements is measured at the outputs of each of the delay elements to determine the phase shift and/or delay through each element. In both the beacon and the distribution cable calibration methods, it is necessary to know the relative phases and/or delays of the calibration signal at the inputs of each antenna and delay element to perform an accurate calibration. Any uncertainties or unknown changes in these relative phases and/or delays produce errors in the calibration measurement and adjustment period.
One conventional antenna calibration system is described in a brief technical paper entitled "Experimental Results From a Self-Calibrating Digital Beamforming Array," by Jeffrey Herd. This paper describes a self-calibrating linear array comprising 32 elemental receivers and a digital beamforming processor which can output 32 custom beams. This system includes a self-calibration system that comprises a calibration source and a calibration feed that is coupled to the receivers. The calibration system uses a closed loop feed network, and the calibration source has two paths to each elemental receiver port. The outputs from the receiver are measured with the test signal fed successively from each side of the loop. Variations in the phase shift and attenuation of the test signal due to the calibration feed cancel out when the measured outputs from both directions are combined. The antenna calibration system referred to above is also described in a technical report entitled "Digital Beamsteering Antenna", by Louis Eber submitted to the Air Force under contract. The report is available from the National Technical Information Service (NTIS) as Rome Air Development Center Technical Report RADC-88-83, June 1988, NTIS No. A200030.
It is therefore an objective of the present invention to provide an improved method and apparatus for calibrating phased array receiving antennas. Another objective of the invention is to provide a method and apparatus for calibrating phased array receiving antennas using a pair of calibration signals to reduce calibration errors. Still another objective of the invention is to provide a method and apparatus for calibrating phased array receiving antennas using a pair of calibration signals applied to the elements of a phased array receiving antenna from opposite ends of a calibration cable connected to the elements. Still another objective of the present invention is to provide a method and apparatus for calibrating phased array receiving antennas which uses a pair of calibration signals of closely displaced frequency and applied to the inputs of the elements of the antenna array from opposite ends of a calibration cable. Another objective of the invention is to provide a method and apparatus for calibrating phased array antennas that is applicable to both narrow band and wide band phased array receiving antennas. Yet another objective of the invention is to provide a method and apparatus for calibrating phased array receiving antennas using a pair of calibration signals of different frequency applied to the inputs of the individual elements of the antenna array from opposite ends of a complementary cable connected to the inputs of the elements of the antenna array.